Selective circuit arrangement



Dec, 1, 1931,

L. c. ROBERTS SELECTIVE CIRCUIT ARRANGEMENT Filed June 21. 1929 ATTORNEY 1' mission through a fmir-fmembe Patented Dec. 1 931 1' LELANDQ. OBERTS,:OFEEOWAG jfiW JnRsEm-nssIGNoR To MER ICANTELEPHGNE AND. TELEGR PH;ooMrANY,'-'A1oon1 o3ArIoN OF NEW YORK] This invention relates to s eI'ctive circuits and more particularlyto selectivecircuit farrangem'ents' for 7 freely transmitting a band offrequencies while: suppressing f l frequencies outside of theban'd'.

One of the primary (injects of the inven tion consists in providing affilter of the band pass type of economical design andha'ving a sharp cut-off at the extremities of' thefrequency band to be transmitted.{According ly, in the' arrangeme'nts of the invention,

there is combined in a circu it' a bandpass fill-- ter and two band suppression filtersfeach oft-he latter being designed to suppress bands of frequencies atthe extremities of the frequ ency band which the'bandpass is de i n d to freely tr mit I In comhinati' n of selective circuits comprising thisiinvention, the band-passfilter utilized may be of the wellkno'wjn Cempbell 1 type, which isfmade up of sectionsic'omprising series and shunt impedance elements.

Such a filter oneratesto"discriminate" between the .band ,or bands of. frequencies.

passed and the bandor 'bands offrequencies sunoressed bv reason of the attenuation propert es of th filter-j.' The."two.band suppress sion fi te s difier from the Campbell tyii fili M6 terin th their selective functions'donot fdeQ Dend noon the impedance looking; intogthe filter. On contrary... a band s electingh'otion isiobtai ne'd in" fthis .tvbe filteriby' tak ini r advantage of the fact hat whose members are may be made extremely"sin all those ifrequen .cies for. which the networlris approximately imbalance and thejlo'ss iscomparatively at frequencies for which thenetworlr is ung' balanced. This is accomplishedbv connect ing in; tandem with an" ordinary gb'and pass filter. whose cut-off it is fdesiredlto make sharper. two'balanced circuits; (hie? of these balanced circuit-s is 'approXimately balance for a hand of frequencle'snear the lower edge of theband which it is desirate trans mit and the other in balance'forfa' band'near.

- the upper edge. bridge one arm of 'which' contains. "a rsonant "circuit", may be usedp'A more desirable arrange- 1929.1 seer m; 37 2,69 6.

ment isthat using thehybrid coil-commonly employed telephone repeater circuits;

' A particular instance in' which the (use. of

the com-binationof selective circuits of this invention would be particularly desirable wonldbe in connection with a voice' frequency carrier'tel'egraphsystem wherein it might be 'desirabl'e'to utilize. higher carrier frequencies than heretofore; It has been the practice'to utilize band passfilters of the Campbell type toprovid'e selectivity with respect to the voice frequency-carrier channels of such a "system; .However',as the frequencies ofth'e ca rrier'channels increase, it becomes exceed mgly -.d1fiicult and expensive to deslgn' "the are utilized. Infact, such filters become com meroially impracticable'with a given band width after acertain frequency limit 'hasbeen passed. However, the combination of selectl-ve circuits of .this 'invention is capable of bandpass filters ofth'e Campb ell typewhich commercial use upfto a much higherfrequency" limit. Accordingly, with the larfrangements of this invention, a great many -more"channels are commercially usable in, such a'systemtlian heretofore. It'is pointed out tliat the foregoing is an illustration. of

only one ofthe many utilizationsithat may be r i made of the combination of selective'a'rrange.

ments of the inventioni ,Other objects and features o-f th'e," invention will appear more fully fr l i the. detailed despription thereof hereinefter givem" V y; The invention may be inoreifully stood from the following description, 7 to getherj w'ith the accompanying drawingsjin the Figures 1, 2,3, and 4 in? wh heinv m ti'on is illustrated. Fig. "1' is' a circuit: diagram embodying fa preferred. form offthe invention.=-'Fig. 2'is a modification thereof.

IniFigs. 3'and 4 are shown curves illustrating the principles of operation of the arrangements of the invention. They 'illustrateina general way, the arrangementsshown in both to m l ke pa 1! 1 l e nected by the combination of selective arrangements ofthe invention. This combination comprises a bandpass filter of the Camp- 7 i belltype and two band suppression filters, or

hybrid coil suppressors,.and the combination operates as a band pass filter with narrow v bandwidth and sharp cut-off. In line section'l isshown a band pass filter 2 of the Campbell type comprising sections of series and 1 shunt inductance and capacity: This" band pass filter for commercial reasons might not have a sharp enough cut-E and accord: I ingly it will be connected to a hybrid coil; arrangement 3. The balancing windings of this hybrid coil arrangement may be connected to a resistance 2t and the line windings maybe connected to a; filter or resonant circuitcomprising the inductance 5 and condenser 6, The resistance of the coil 1s -1n-V;

dicated as a separate unit. At the resonant frequencythe impedance of the resonant circuit andthe impedance of the circuit includ-' ing' the resistance a are equal" and hence no energy is transferred through the ,hybrid coil. These i'mped ances are unbalanced for irequencies above or below th s resonant irequency',qand hence the loss through the hybrid coil-becomes increasingly less'as the departurej from the resonant frequen'cy. becomes greater, 7 It might be desirable to introduce loss in a narrower bandof frequencies than ltheabove arrangement allows.- This maybe pass filter, such as: filter 2.

done by connecting a resonant circuit, such ascircuit-l2 of F g. 2, to the balancing windings in place of the pure resistance i. -Asec- :ond hybrid coil suppressor 7, would; be con nectedin the circuit and'would have con,- nectedtothe line windings a resonant cir cuit comprising induct-ance 9 and condenser 10. {The balancing windings of-this arrange;-

ment would'be connected toa resistance '8.

Connected to the midpoints of thishybrid .coil arrangement is the line section 11. o

The two hybrid coil suppressors would; be designedjto suppress abandof frequencies at each edge of the free transmission band desired, such as the band passed by; the filter 2. For purposes of illustration, let "it-be as:

s ed hat l a ran e t i br u 1n a AYOICB frequency carrier telegraph sys tie-m, and that-the carrier channel under con.-

- s'ide ra'tion was 2465 cycles and with channels spaced'ZO cycles apart. Under such conditions, oneLhyb-rid coil suppressor'wouldbe adjusted with its resonant frequency about .2550 to 2560cycles and the other hybrid coil suppressor would ;be. adjusted j with its resonant frequency about 2370 to 2380 cycles.

jgnIn Fig. 3 is acurveA showing the at-' tenuation frequency characteristics of a band The curves B and-B show the attenuation frequency-char acteristics of the hybrid coil suppressors, such as 3 and 7 If it is desired to narrow the band.

toirbe passed by filter-2 and to provideit with a sharper c'ut-ofl, the hybrid coilsuppressors 3 and 7 will be adjusted with their resonant frequencies at f and f or in other words, at either'edg'e' of the frequencyband passed by the filter 2. The resultant of the combination is shown by the attenuation frequency curve C of Fig. 4;

While the inventionhas been disclosed as embodied in certain specific arrangements which are deemed desirable, it is understood that git is capable of embodiment in many and other widely varied forms without departing from the spirit of the invention as defined by the appended claims.

Whatisclaimed'is: o 1.;A- 'combination ofgselective circuits "comprising aband pass ,filter and-two'band sup- 1 path will be sharplyfdefinedjbetween the neighboring edgesofithe two suppression bands. y 5 h V ',2. A transmissionline comprising two line sections interconnected by acombination of selective circuits,sai d' combination comprise ing a band pass filter and two bandsuppres sion filters of the balancedhybrid coil sup-- pressor type-arranged a common path, one

suppression filter-being designed to'suppress a band "of frequencies at one extremity of the band passed by said first mentioned filter and .the other suppression filterbeing designed tosuppress a band of frequencies at the'other extremityofthe band passed by said first-men- I tioned filter, whereby the band passed over the commonpath be sharply definedbetweensthe neighboring edges of thesuppresrsionbands, o x

7; =3, A} transmission ,line comprisin g two I line sections interconnected by a combination of selective circuits. saidgcombination compris ing' a filter, designed to pass s n-arbitrarily chosen band of frequencies and two band suppression filtersrof the hybrid coil suppressor type, arranged in;a common transmission path, one of said hyblfid-. coil suppressortype filtersvbeing designed to; suppress aband of frequencies at one extremity of said arbitrarilyj chosen band,andithe-other hybrid coil suppressor type: filterbeing designed to suppress a band offifreq encies: at the other extremity of the band passed by said first mentionedfilter,'whereby the band passed over the common path will be sharply defined betweenithe neighboring edges of the two suppression bands. j l

. 4'; A transm1ssion line comprising two line sections interconnected by a combination of selective circuits, i said combination compris pression filterszof the hybrid coilsuppressor ing a filter designed to pass an arbitrarily 7/ chosen band of frequencies andtwo band suptype arranged in a common transmission path, one of said hybrid coil suppressortype filters being designed to suppress a band of j frequencies at one extremity of said arbitrarily chosen band, and the other of said hybrid coil suppressor type-filters being .de--

signed to suppress a band of filters at the first-mentioned filter, whereby the i band passed over the common path will be sharply defined between the neighboring edges of the two suppression bands, and each suppression filter comprising a hybrid coil balancing circuit having a resonant circuit connected to to prevent transmission of said first mentioned range of frequencies therethrough but "is unbalanced to permit transmission 0ffrequencies outside of said range. I

' In testimony whereof,I -have signed my name'tothis specification this 20th day of June 1929.

other extremity of the band passedby said I the line terminalsof said coil and an 'impedance element which is substantially con stant with frequency connected to the balancing terminalsof the hybrid coil;

LELAND GROBERTS.

5. A transmission line comprising twoline having a resonant circuit connected to the 7 line terminals of said coil and an impedance element which is substantially constantwith' frequency connected to the balancing wind-f T ings of said hybrid coil, one of said resonant clrcuits bemg deslgned for resonance at frequencies at one extremity of said arbitrarily chosen band of frequencies, and the other resonant circuit being designedfor resonance at frequencies at the other extremity of said band, whereby the band passed over, said common path will be sharply defined between frequencies near said-resonant frequencies.

6. A transmission line comprising two line sections interconnected by a combination of selective circuits, said combination comprising a filter designed to pass an arbitrarily chosenband of frequencies and two band suppression filters of the hybrid coil suppressor type, the said hybrid coil suppressor'type filtersbeing designedto suppress bands of f frequencies at the opposite extremities" ofthe said arbitrarily chosen'band, andeach of said hybrid coil suppressor type filters comprising a. circuit arrangement, a network associated with said circuit arrangement said" network having fa. definiteimpedance in one range of frequencies {and a materially 'dlfferent impedance outside said range, and an impedance device associated with said circuit arrangement in balancing relation to said net 7 Y work, said impedance device'having substan V tially the same impedance as said network in said first mentioned range of frequencies and a different impedance outside said range,

"whereby said circuit arrangement is balanced l 

